Upon returning to the Normandy after the previous mission, you receive a private communiqué from an asari representative on the Citadel. She requests your presence at the Citadel, so go there first to pick up your next mission.
Go to Councilor Udina’s old office and speak with the asari Councilor. She reveals the location of a secret artifact that may help reveal the answer to the Catalyst question. As you might expect, the asari Councilor is careful to reveal the specifics about the artifact.
If you press her, however, you can get more information. Reply in accordance with your Paragon/Renegade leaning. If you want Paragon points, be agreeable. If you’re a Renegade, confront her.